The Seeds of Life: From Aristotle to da Vinci, from Shark’s Teeth to Frog’s Pants, the Long and Strange Quest to Discover Where Babies Come From

By Edward Dolnick

Basic Books, 2017; 320 pages; $28.00

The first person to discover where babies come from, strictly speaking, was a German biologist named Oscar Hertwig. In 1875, working at his microscope in a Naples, Italy laboratory, Hertwig dabbed a drop of sea urchin semen against the gelatinous surface of a sea urchin egg. “Moments later,” according to science journalist Edward Dolnick, “the nucleus of the sperm cell came into view, inside the egg…and then—before Hertwig’s eyes—the two nuclei fused into one. No one in history had ever seen the process of fertilization play out.”

Forgive me for giving away the ending, but Hertwig’s revelation occupies only the last three pages of a rich and entertaining history of conceptions and misconceptions about the reproductive process.

Was the woman merely a passive field in which the male planted a “seed” carried somehow in semen? Many believed this; in fact, the word semen comes from the Latin word for seed. Or did the woman produce an egg, as did many other creatures, with “semen” only playing a role as a mysterious stimulant to the development of the embryo? William Harvey, best known for his work on the circulation of the blood, argued thus in a 1651 opus, Disputations Touching the Generation of Animals, whose title page bore the Latin motto Ex Ovo Omnia (Everything comes from the egg). Informed opinion through the 1600’s and the 1700’s was split between these two opposing camps.

A major source of this confusion was that, for humans at least, egg and sperm were so small. Early microscopists had observed sperm cells in semen as early as the 1600’s, but they mistook the wriggling swimmers as parasitic squatters, akin to the little creatures seen in drops of pond water. And searches for eggs in newly impregnated women, or in dissected ovaries, failed to turn up anything convincing until 1827, when Karl Ernst von Baer, an Estonian biologist, observed a “small yellow spot in a little sac” in the ovaries of a dog.

An even bigger obstacle, however, was the difficulty of imagining living organisms as self-assembling communities of cells. Champions of both semen and egg tended to view them as bearers of miniature creatures, rather than sets of do-it-yourself instructions. The prevailing wisdom was that God not only created all living creatures during the first week of Genesis, but that he created all generations of all creatures back then. By this logic, Eve’s egg (or Adam’s semen) contained miniature humans who in turn contained miniature humans, and so on like an infinite set of nesting dolls. The logical difficulties of this theory were legion—how did it account for the fact that children often resembled both parents, for instance—but the notion of pre-existence, as it was known, dominated throughout the 1700’s.

It’s easy to smile at such quaint ideas in an era when the birds and the bees, not to mention DNA and gene modification, are part of everyday discourse. But today’s common knowledge, as Dolnick shows, owes everything to the anatomists, microscopists, and clever experimentalists who labored for centuries over life’s greatest mystery.